Depositional facies of the subsurface Neogene Surma Group in the Sylhet Trough of the Bengal Basin,Bangladesh: record of tidal sedimentation

The Bengal Basin, in the north-eastern part of the Indian subcontinent, contains a thick (± 22 km) early Cretaceous-Holocene sedimentary succession. The Neogene succession in the Sylhet Trough of the basin reaches a thickness of more than 6 km of which the Surma Group contains important sandstone reservoirs. Lithologically, the group consists of a succession of alternating shales, siltstones, sandy shales and sandstones, with minor conglomerates. This research work is a sedimentological analysis of the subsurface Neogene succession encountered in the petroleum exploration wells in the Sylhet Trough of the Bengal Basin. Detailed lithologic logs of the cores, based on considering texture and sedimentary structure, permit a subdivision into eight lithofacies, e.g., a shale-dominated facies, interbedded fine sandstones and mudstones, ripple-laminated sandstones, parallel-laminated sandstones, massive sandstones, cross-bedded sandstones, cross-bedded sandstones with pebble/granule lag and conglomerates. Characteristic sedimentary structures of the Surma Group, such as flaser-, wavy- and lenticular-bedding, bipolarity of ripple cross-stratification, evenly laminated sand/silt-streaked shales, reactivation surfaces within cross-bedded sandstone sets, mud-drapes on foreset laminae and herringbone cross-stratification as well as small-scale vertical sequences (several fining-upward cycles) are diagnostic for tidal influence. On the basis of the lithofacies associations and prograding character of the deposits revealed from the electrofacies associations, the Surma Group sediments have been interpreted as representing deposits of tide-dominated deltaic depositional setting.     

Signature of a paleogene submarine-fan from the Jenam Formation,Barail Group,Assam-Arakan orogen,northeastern India

Geological domains in northeastern India evolved though time after the rupture of the Gondwanaland. Collision of the Indian and the Burmese plates took place during the middle part of Cretaceous. Evolution of the Paleogene Barail trough, Neogene Surma and Tipam Groups took place gradually. The Barail trough originated at the active margin of the Indo-Burmese plate convergence. Previously, the Barail sedimentary wedge was interpreted to have been deposited in a deltaic to shallow-marine environment. Latter studies have proposed a new depositional model. The Barail Group, a part of the Assam-Arakan Orogen comprises the lowermost Laisong Formation, middle Jenam and upper Rinji Formation. Most of the Paleogene strata of the Barail Group carry imprints of a deep-water submarine fan near an active subduction zone. A deep-water proximal- to mid-fan depositional setting has been ascribed to the lower Laisong strata. Facies analysis of the extensive exposures of the Jenam Formation, near the Jenamghat, Assam, have enabled a detailed reconstruction of a proximal to mid-part of a submarine fan under an overall influence of turbidity currents and debris flow, ultimately evolving into turbidity currents. The Jenam sedimentary wedge bears tell-tale preservation of olistostromes (chaotic facies) with abundant volcaniclastics and basic rock fragments, massive sandstones, conglomeratic sandstone to sandstone-siltstone-shale and sandstone-siltstone-mudstones with profuse turbidites. Facies attributes amply signal the inherent instability of the Barail trough owing to its location near an active subduction zone. The Jenam suite of rocks containing a strong impress of volcanogenic materials along with quartzo-feldspathic rocks were mixed up by turbidity currents and shed into the submarine environment as the Jenam deep-water turbidites.     

青藏高原北羌塘地区晚三叠世地层展布和沉积型式

北羌塘盆地地处拉竹龙-金沙江缝合带和双湖构造混杂岩带之间,自北向南可划分出5个沉积相带/岩石地层单位：以砂泥质复理石-洋岛、岛弧型火山岩-大理岩岩石组合沉积为特征的若拉岗日群,以深水复理石盆地相沉积为特征的藏夏河组,以深水暗色细碎屑岩盆地相沉积为特征的结扎群,以开阔台地相/缓坡相碳酸盐岩沉积为特征的菊花山组,以三角洲相含煤碎屑岩系沉积为特征的土门格拉群.晚三叠世北羌塘盆地显示为南缓北陡的箕状沉积格局,盆地内充填物为南薄北厚的楔形沉积体,且双物源、沉降中心和沉积中心不一致,表明其具有前陆盆地的一系列沉积特征.     

广西罗城—鹿寨地区石炭系沉积相类型及沉积环境演化

本文据区内6个石炭纪地层剖面的岩性组合,垂向层序及生物组合特征的研究,将沉积相区分为陆源碎屑潮坪沉积、三角洲沉积、浅水开阔台地和极浅水局限台地碳酸盐沉积以及半深水台盆沉积五种类型。研究表明,在石炭纪不同时期的沉积环境具有不同的组合特点。早石炭世沉积类型比较复杂,具有陆源碎屑潮坪-三角洲-开阔台地-台盆环境组合形式;而中晚石炭世则主要为极浅水局限台地环境,总体上表现出向上变浅的环境演化特点。本文指出台地向上变浅的机理是在总体浅进超覆的背景下由碳酸盐沉积物的快速沉积造成的。     

Interaction between Downslope and Alongslope Processes on the Margins of Daihai Lake,North China: Implication for Deltaic Sedimentation Models of Lacustrine Rift Basin

Daihai Lake, a modern lacustrine rift basin, located in Inner Mongolia, North China, serves as an important modern analog for understanding deltaic depositional processes in an active rift setting. Two of the deltas (Yuanzigou delta and Bulianghe delta) on the margins of Daihai Lake were surveyed to compare and contrast stacking patterns using aerial photographs, field trenching and sediment sampling. Shallow cores and trench data collected from the margins of Daihai Lake indicate that a variety of depositional processes have been active since Daihai Lake formed. Two 3-D sedimentation models which employ chronostratigraphic correlation technique were generated. The chronostratigraphic sedimentation models predict and represent the architectures and sand-body continuity of sediments. Stratigraphical coincidence of the broad sheeted drifts and channel erosion suggests a coupling between downslope and alongslope processes. Distributary mouth bars are prevalent in the front of deltas on steeper slopes due to the dominance of down-slope flows. On the contrary, the along-slope currents favor the development of distal bar deposits with sheeted sandbodies on gentle depositional slopes. This study provides an insight into the architecture of complex sedimentary facies associated with highlighting key differences between downslope flows and alongslope currents. The distribution of sand within these deltas is of particular interests, with applications in understanding the architecture of hydrocarbon reservoirs formed in lacustrine rift basin.     

Fluvial braidplain to shallow marine transition in the early Neoproterozoic Morar Group,Fannich Mountains,northern Scotland

The early Neoproterozoic Morar Group in northern Scotland forms the lower part of the Moine Supergroup, deformed and metamorphosed within the Precambrian Knoydartian and Lower Palaeozoic orogenies. It has remained uncertain whether it was deposited in a shallow-marine ‘failed rift’ basin within Rodinia or a foreland basin on the margin of the Grenville orogen, which is important to determine for tectonic reconstructions. In that context, we assess the sedimentology, depositional environment and tectonic setting of the middle part of the Morar Group in the Fannich Mountains. A ca. 4–6 km thick fining-upward facies succession contains three psammite dominated lithofacies (LF): LF1, at the base, contains amalgamated and multi-storey sets of trough and tabular cross beds, which passes upwards into LF2 consisting of trough and tabular cross-bedded units arranged in coarsening and fining-upward sequences with minor pelitic rocks. The stratigraphically highest lithofacies, LF3, contains finer and more complex coarsening-upward packages of rhythmically interbedded pelite and psammite. Palaeoflow develops from broadly unimodal NW–NE flow in LF1, to weakly bimodal NW–SE flow in LF3. The data indicate that this part of the Morar Group records deposition in a distal fluvial braidplain to tidally influenced shallow-marine setting. All facies and palaeocurrent transitions are gradual and occur systematically over many hundreds of metres vertically; such characteristics are incompatible with a rift-basin setting. We suggest that, instead, deposition occurred in a transition between a foreland basin to the Grenville orogen and a marine basin associated with the Asgard Sea between Baltica and Laurentia.     

Hyperspectral logging of middle Cambrian marine sediments with hydrocarbon prospectivity: a case study from the southern Georgina Basin,northern Australia

The Georgina Basin is a Neoproterozoic–Paleozoic basin that spans parts of the Northern Territory and Queensland in northern Australia. The basin is prospective for petroleum, phosphate and base metals (copper, lead and zinc). The Dulcie and Toko synclines in the southern part of the basin are prospective for petroleum, where a thick Cambro-Ordovician succession of marine carbonates hosts several source rocks and associated oil and gas shows. The key source rock units occur within the middle Cambrian Narpa Group, including both the Thorntonia Limestone (Series 2 and 3) and the Arthur Creek Formation (Series 3). The base of the Arthur Creek Formation is characterised by organic-rich ‘hot’ shales (associated with a prominent gamma spike in well logs) that have been targeted by petroleum explorers for both conventional and unconventional oil and gas. For this study, hyperspectral logging data collected by HyLogger? instruments were evaluated from 13 wells in the southern Georgina Basin, including petroleum, mineral and stratigraphic wells. Formation boundaries are commonly (but not always) characterised by distinctive changes in mineralogy, as determined by spectral and X-ray diffraction data. Key source rock units in the southern Georgina Basin were characterised and mapped in terms of their mineralogy, and other spectral properties (e.g. Short-Wave Infrared (SWIR) reflectance and spectral contrast). Interpretation of the hyperspectral data alongside wireline log data supports the differentiation of two successions within the Arthur Creek Formation that are each characterised by basal organic-rich shales, previously distinguished on the basis of biostratigraphic and well-log data. The older succession in the Dulcie Syncline is spectrally characterised as being quartz and carbonate dominated. The younger succession, distributed across the eastern part of the Dulcie Syncline and fully across the Toko Syncline, is spectrally characterised as quartz and carbonate dominated, with variable white-mica contributions. Key associations are observed between the HyLogger mineralogy and geophysical-log data. Peaks in the gamma log intensity in the middle Cambrian sediments commonly correspond to elevated measured total organic carbon contents, decreased carbonate contribution, SWIR reflectance and spectral contrast, and relatively increased proportions of white micas and quartz. This study demonstrates that HyLogging data can provide an improved understanding of the sedimentological, mineralogical and diagenetic characteristics, as well as associated spatial heterogeneity, of prospective hydrocarbon formations in sedimentary basins.     

Geochemical evolution of the Tertiary succession of the NW shelf,Bengal basin,Bangladesh: Implications for provenance,paleoweathering and Himalayan erosion

Whole rock major, trace and rare earth element (REE) compositions of Paleogene to Neogene sedimentary rocks of the NW shelf succession (Province 1) of Bangladesh contain a record of interaction of the India and Asian plates, Himalayan tectonism, and climatic development. Analyses of 66 sandstones and mudrocks from the Tertiary succession of Bangladesh were made to examine provenance, source weathering, and the influence of paleoclimate and tectonism. The sediments display linear geochemical trends due to quartz dilution, and varying quartz–clay ratios produced by hydrodynamic sorting. Chondrite-normalized REE patterns for both sandstones and mudrocks from different groups are similar to upper continental crust, with moderate to high LREE enrichment (lithotypes within formations average La_N/Yb_N 5.31–11.41) and marked negative Eu anomalies (Eu/Eu^* 0.51–0.69). Based on geochemical criteria the succession can be divided into three parts (Jaintia; Barail–Surma; and Dupi Tila). Very high silica contents in Jaintia Tura sandstones and high Chemical Index of Alteration (CIA) indices in Kopili mudrocks (Fe-shales) suggest derivation from a deeply weathered and stable cratonic source (India). The Tura sandstones are interpreted as first-cycle quartz arenites, produced while the Indian plate drifted across equatorial regions during the Paleocene–Eocene Thermal maximum (PETM). The Barail–Surma and Dupi Tila sediments were derived from a felsic orogen (the Himalaya). The Barail–Surma sediments were mainly derived from the Trans Himalayan Batholith and associated granitoids, with significant contribution from the Lesser Himalaya. Mafic input is also evident, probably from intraoceanic arc material within the Himalaya. Barail mudrocks have uniformly high CIA values (92–95), suggesting intense steady-state weathering of their proto-Himalayan source, and warm and humid climate. In contrast, CIA values of Surma mudrocks range from 66 to 93 (average 84), suggesting non-steady state weathering related to active uplift in the Himalaya. The Dupi Tila sediments were derived from a more felsic Lesser Himalaya source, with significant contribution from the Trans-Himalaya and very little or no ophiolitic or arc material. Dupi Tila mudrocks have CIA ratios of 62–99 (average 72), also indicating non-steady state weathering in the rising Himalayan source. Geochemical compositions of the NW shelf sediments are comparable to coeval successions in the Surma basin (Province 2) of Bangladesh and the Siwaliks (India), indicating similar source. Evolution of the Indian monsoon and associated high precipitation caused intense chemical weathering of the Surma and Dupi Tila source, despite rapid uplift. The Surma Group thus bears the signature of evolution of the Asian monsoon in the Bengal basin at 21 Ma, simultaneous with the development of the East Asian monsoon. This supports proposals that both monsoon systems developed at the same time.     

Sedimentary facies analysis and seismic interpretation of deep-water reservoirs, “PAC” field,offshore depobelt,Niger Delta basin

A high-resolution study involving the integration of cores, wireline logs and reflection seismic dataset was used to unravel the facies assemblages, reservoir quality and hydrocarbon potential of E1 and H9 reservoirs in the “PAC” field, Offshore Niger Delta. Facies analysis of the cored PAC-14 well shows six lithofacies: massive mudstone, parallel-laminated mudstones with sideritic bands, fine-grained parallel-laminated sandstone, medium-grained parallel-laminated sandstone, fine-grained ripple laminated sandstone, and coarse massive sandstone. These lithofacies were grouped into five facies associations: channel story axis (CSA), channel story margin (CSM), inter-channel thin beds (ICTB), mud-rich thin beds (MRTB) and injectites (INJ). These facies associations are typical of a confined channel and basin floor fan deposit of a deep-water turbidite depositional environment. Reservoir unit E1 was dominated by the facies association CSA while the H9 reservoir consists mainly of the facies associations MRTB and INJ. The observed sand injectites are suggested to have formed due to overloading of compacted sands leading to upward remobilization of sand into the overlying shale. Seismic horizons and faults were mapped to understand the structures, trend and reflections within the study area. Results from petrophysical volumetrics estimated the net hydrocarbon pore volume for the E1 and H9 reservoirs as 456 MMBBL and 378 MMBBL, respectively. Exploration for deep-water reservoir mainly targets high amplitude and bright seismic reflectors and ignores the low amplitude reflectors (dim loops) which are typically interpreted as non-reservoir units. However, this study shows a low-amplitude reflector containing medium-grained sand injectite unit, which has high porosity (31.7%) and permeability (4472 mD) values, and contains recoverable hydrocarbon.     

Pleistocene glaciolacustrine sedimentation in a tectonically active zone, Kleszczów Graben, central Poland

The Kleszczów Graben in central Poland was formed by late Oligocene to Middle Pleistocene extensional tectonics. During the Pleistocene it was infilled with a 200 m thick sequence of predominantly glacial sediments. Four distinct formations of Elsterian and Saalian age are identified, each containing 15–40 m of glaciolacustrine strata. The boundaries between formations are marked by erosional surfaces and, in part, by angular discordances caused by tectonism. Glaciolacustrine sedimentation was tectonically controlled: the thickness of the sequences in the graben are three to five times greater than outside the area of fault-controlled subsidence. Deposition in the proglacial lakes was controlled by differential subsidence rates within the basin: deep-lake facies (varved clays) were deposited in sub-basins with high subsidence rates and deltaic to shallow-water facies accumulated in areas of moderate subsidence or occasional uplift. These variations led to the development of a very complex, ‘mosaic’ of lateral facies relationships, suggesting that several sub-basins with differing subsidence rates were present. The Vertical successions show proximal-distal sequences typical of glacier-fed lakes that have limited contact with the ice sheet. However, gravity flow facies are very common, and occur both in the shallow- and deep-water deposits. These deposits are interpreted to have been formed adjacent to active fault scarps which bordered the lake basin. Although several distinct phases of glaciolacustrine sedimentation occurred during the history of trough infilling, the location of the areas of high subsidence varied through time.     

Depositional environment and provenance of Middle Siwalik sediments in Tista valley,Darjiling District,Eastern Himalaya,India

The frontal part of the active, wedge-shaped Indo-Eurasian collision boundary is defined by the Himalayan fold-and-thrust belt whose foreland basin accumulated sediments that eventually became part of the thrust belt and is presently exposed as the sedimentary rocks of the Siwalik Group. The rocks of the Siwalik Group have been extensively studied in the western and Nepal Himalaya and have been divided into the Lower, Middle and Upper Subgroups. In the Darjiling–Sikkim Himalaya, the Upper Siwalik sequence is not exposed and the Middle Siwalik Subgroup exposed in the Tista river valley of Darjiling Himalaya preserves a ~325 m thick sequence of sandstone, conglomerate and shale. The Middle Siwalik section has been repeated by a number of north dipping thrusts. The sedimentary facies and facies associations within the lithostratigraphic column of the Middle Siwalik rocks show temporal repetition of sedimentary facies associations suggesting oscillation between proximal-, mid- and distal fan setups within a palaeo-alluvial fan depositional environment similar to the depositional setup of the Siwalik sediments in other parts of the Himalaya. These oscillations are probably due to a combination of foreland-ward movement of Himalayan thrusts, climatic variations and mountain-ward shift of fan-apex due to erosion. The Middle Siwalik sediments were derived from Higher- and Lesser Himalayan rocks. Mineral characteristics and modal analysis suggest that sedimentation occurred in humid climatic conditions similar to the moist humid climate of the present day Eastern Himalaya.     

Eocene fan delta-submarine fan deposition in the Wagwater Trough, east-central Jamaica

ABSTRACT The Wagwater Trough is a fault-bounded basin which cuts across east-central Jamaica. The basin formed during the late Palaeocene or early Eocene and the earliest sediments deposited in the trough were the Wagwater and Richmond formations of the Wagwater Group. These formations are composed of up to 7000 m of conglomerates, sandstones, and shales. Six facies have been recognized in the Wagwater Group: Facies I-unfossiliferous massive conglomerates; Facies II—channelized, non-marine conglomerates, sandstones, and shales; Facies III-interbedded, fossiliferous conglomerates and sandstones; Facies IV—fossiliferous muddy conglomerates; Facies V—channelized, marine conglomerates, sandstones, and shales; and Facies VI—thin-bedded sheet sandstones and shales. The Wagwater and Richmond formations are interpreted as fan delta-submarine fan deposits. Facies associations suggest that humid-region fan deltas prograded into the basin from the adjacent highlands and discharged very coarse sediments on to a steep submarine slope. At the coast waves reworked the braided-fluvial deposits of the subaerial fan delta into coarse sand and gravel beaches. Sediments deposited on the delta-front slope were frequently remobilized and moved downslope as slumps, debris flows, and turbidity currents. At the slope-basin break submarine fans were deposited. The submarine fans are characterized by coarse inner and mid-fan deposits which grade laterally into thin bedded turbidites of the outer fan and basin floor.     

湖泊细粒沉积特征精细研究：以鄂尔多斯盆地延河剖面长7油层组为例

野外地质露头为精细刻画沉积体内部结构、建立准确地下地质模型发挥着重要作用。以鄂尔多斯盆地延河剖面长7段为例,采用岩石学、野外露头沉积学方法,详细剖析了湖泊细粒沉积的岩相类型、特征、垂向组合及沉积环境。研究结果表明,延河剖面长7段发育平行层理细砂岩相、流水交错层理细砂岩相、浪成交错层理粉砂岩相、沙纹层理粉砂岩相、变形层理粉砂岩相、水平层理(泥质)粉砂岩相、块状泥岩相、水平层理(砂质)泥岩相、水平纹层页岩相9种岩相类型。在结合区域地质特征基础上,研究认为长7段为远源的曲流河三角洲前缘和浅湖-半深湖沉积,进一步细分出7类沉积单元,其中水下分支河道、支流间湾较为发育,水下天然堤、远砂坝、席状砂发育规模较小,浅湖-半深湖沉积只在长72段下部发育,河口坝基本不发育,仅局部可见。对各沉积单元的垂向分布特征进行深入研究,识别出Ⅰ、Ⅱ、Ⅲ、Ⅳ 4类垂向分布形式,其中Ⅰ、Ⅱ组合主要分布在研究区长71、长73亚段,Ⅲ、Ⅳ组合主要分布在研究区长72亚段。剖面相分析表明,长7沉积期整体为一套先变细、再变粗的细粒沉积序列,为曲流河三角洲前缘沉积—浅湖-半深湖沉积—曲流河三角洲前缘沉积。     

Mineralogy and chemical composition of the clay fraction of Neogene shales from the Surma Group in the Bengal basin,Bangladesh

Mineralogical and chemical investigations (<2μm clay separates) of shale samples from the Neogene-age Surma Group obtained from four wells (Habiganj-11, Shahbazpur-1, Titas-11, Titas-15) in the Bengal basin, Bangladesh, were carried out in order to reveal the clay mineral composition as reservoir exploration and exploitation requires a good understanding of the clay minerals. The samples were analyzed by X-ray diffraction (XRD), scanning electron microscope (SEM) and X-Ray fluorescence spectrometry (XRF). Mineralogically, the sub-surface Surma Group shales comprise predominantly quartz, plagioclase, illite, chlorite, kaolinite, with lesser amounts of K-feldspar, dolomite and smectite, and minor to trace amounts of calcite, siderite and pyrite. The chemical composition of the <2 μm clay separates also suggests an illite and chlorite-rich composition. With increasing burial depth, the Surma Group shales are enriched in illite. The gradual decreasing of the smectite clays with depth and ultimate disappearance at greater depths (≥ 3000 m) may have been responsible for the presence of the diagenetic illite. Based on the mineralogical composition it is most likely that the illite-chlorite associations together with quartz and feldspar were predominantly detrital in origin and thus reflect the presence of a rapidly-rising source terrain not subjected to intense weathering.     

Evolution of an early Proterozoic foreland basin carbonate platform, lower Pethei Group, Great Slave Lake, north-west Canada

The Taltheilei, Utsingi, McLean and Blanchet formations form a 175–390 m thick carbonate platform-to-basin succession in the lower part of the PaleoProterozoic Pethei Group, preserved in the eastern arm of Great Slave Lake. Carbonates accumulated along the south-east margin of the Slave Craton within a foredeep formed during the collision of the Slave and Churchill Cratons. The rocks include eight, predominantly microbial, carbonate facies that comprise five facies associations representing (1) shallow-water rimmed shelf, (2) shallow-water open shelf, (3) shallow-water ramp, (4) upper slope and deep ramp, and (5) lower slope and basin plain environments. Microbialite facies grew by organically mediated precipitation of spar and micritic cement and trapping and binding of lime mud. These wholly subtidal facies typically reflect progressive shallowing and changing geometry of the lower Pethei sea floor, from ramp, to open shelf, to shallow rimmed shelf, with associated slope and basin plain deposition. Repeated relative sea-level changes influenced platform growth. This resulted in five shallowing upward packages; each separated by an incipient drowning event of varying magnitude. Antecedent topography and the size of the preceding drowning event strongly influenced the initial growth of each interval. This repeated pattern is attributed to interaction between (a) the inherent tendency of microbial carbonates to aggrade vertically, (b) changing sedimentation rates and (c) readjustments of relative base level. The lower Pethei succession is one of few PaleoProterozoic examples of carbonate platform growth within a foreland basin. It has (1) a low gradient profile, (2) extensive slope and basin plain carbonate production and sedimentation, (3) no ooids, (4) minor terrigenous clastic sediments, and (4) a mobile, submergent shelf rim lacking substantial carbonate sand shoals.     

第三纪惠民盆地的沉积相和沉积大地构造演化

山东惠民盆地中所堆积的厚达7000米的第三系地层,主要是陆源碎屑沉积,其次是碳酸盐沉积。解释这些沉积物的沉积环境,显然有助于预测盆地中有利的生、储油相带。我们在分析多项指相标志的基础上,首先总结了主要沉积相的沉积序列和沉积模式,然后概略地描述了不同时期盆地中沉积相的展布,最后将盆地的构造演化与沉积特征相结合,进行沉积大地构造方面的探索。     

Identifying water-quality trends in the Trinity River,Texas, USA, 1969–1992, using sediment cores from Lake Livingston

 Chemical analyses were done on cores of bottom sediment from three locations in Lake Livingston, a reservoir on the Trinity River in east Texas to identify trends in water quality in the Trinity River using the chemical record preserved in bottom sediments trapped by the reservoir. Sediment cores spanned the period from 1969, when the reservoir was impounded, to 1992, when the cores were collected. Chemical concentrations in reservoir sediment samples were compared to concentrations for 14 streambed sediment samples from the Trinity River Basin and to reported concentrations for soils in the eastern United States and shale. These comparisons indicate that sediments deposited in Lake Livingston are representative of the environmental setting of Lake Livingston within the Trinity River Basin. Vertical changes in concentrations within sediment cores indicate temporal trends of decreasing concentrations of lead, sodium, barium, and total DDT (DDT plus its metabolites DDD and DDE) in the Trinity River. Possible increasing temporal trends are indicated for chlordane and dieldrin. Each sediment-derived trend is related to trends in water quality in the Trinity River or known changes in environmental factors in its drainage basin or both. Received: 6 October 1995 · Accepted: 13 October 1995